Bubbles, bubbles, my bubbles!

The title of course comes from Bubbles, the lovable and high-strung yellow tang in Disney’s “Finding Nemo”. I was inspired to write this because one of the most common questions I get regarding the reef aquarium is – somewhat surprisingly to me – about the bubble algae, rather than about the animals. Bubble algae are considered to be single-celled algae in the genera Valonia, Ventricaria, Boergesenia or Dictyospheria. Whether or not they are is apparently in question because of some unusual features described by Shepherd et al. (2004). In fact, there has been a considerable body of work on the ultrastructure and function of the cell wall of some species because of this. There is less information available on how to control this invasive alga, although the topic is frequently discussed online.

Valonia species are marble-shaped, whereas Ventricaria spp. look like small balloons. Boergesenia spp. Are elongated, and look somewhat like green hot dogs. Dictyospheria spp. are smallish and often grow in clusters. They have in common that once in a reef aquarium they are virtually impossible to get rid of. The species in the UNBC reef tank is most likely Ventricaria (formerly Valonia) ventricosa (Olsen and West 1998). It is dark green, often with a metallic shimmer caused by refraction. Bubbles start off small, but eventually reach a size of about 3-4 cm long and perhaps ¾ of the length in diameter. They attach firmly to any hard substrate, including coral skeletons. This sometimes causes corals to be dislodged or overgrown. When mature, the bubble will burst (no pun intended) and release spores, leading to the establishment of new bubble algae if the spores end up on suitable substrates.

One of the most common recommendations for combatting bubble algae is through the introduction of emerald crabs, Mithraculus sculptus. While these crabs consume some

small bubble algae, they are omnivores, and the effect will largely depend on what alternative food sources are available to them (Figueiredo et al. 2008). Another frequent suggestion is the lettuce “nudibranch”, Elysia crispata. This sacoglossan sea slug feeds on a number of algae species, and has been reported to use kleptoplasty (acquisition of functioning chloroplasts for photosynthesis) as a means of producing energy in the absence of food, although this has been questioned in recent papers (Christa et al. 2014a,b). I have added both emerald crabs and lettuce sea slugs to the UNBC reef tank on numerous occasions, but I can’t say that I have seen any improvement in the bubble algae density as a result. Many sea slugs are quite specialized in terms of their food, and there is at least one species that that appears to specialize on bubble algae. This fascinating example is Ercolania kencolesi, a species that manages to get inside the bubble of Boergeseniaspp. without bursting it (Grzymbowski et al. 2007, Händeler et al. 2009).

A related sea slug, Ercolania endophytophaga feeds inside the basal cells of Struvea plumosa in the wild, but also on Valonia spp. in the laboratory (Jensen 1999). According to a post in the Sea Slug Forum, it has also been found inside Valonia on the Great Barrier Reef (http://www.seaslugforum.net/showall/ercoendo).

Since emerald crabs are rather inefficient for the most part, and the sea slugs are difficult to get hold of and keep alive, bubble algae will likely be a feature of the UNBC reef tank for the foreseeable future.

Jensen, K.R. 1999. A new species of Sacoglossa (Mollusca, Opisthobranchia) from Rottnest Island, Western Australia. Pp. 377-383 in D.J.Walker and F.E.Wells (Eds). The Seagrass Flora and Fauna of Rottnest Island, Western Australia. Western Australian Museum, Perth .